Inspection Technical Tip
S U B PA N E L G RO U N D I N G B A S I C S
By Michael Casey & Barry Stone COA—The Right Direction℠
Photo COA
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COA– Inspection
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Inspection Technical Tip
S U B PA N E L G RO U N D I N G B A S I C S
By Michael Casey & Barry Stone COA—The Right Direction℠
There are few subjects in the field of home inspection that raise as many questions, debates, and
uncertainties as subpanel grounding. This article will address many of those concerns and provide
some clarity on this vital aspect of electrical safety. The points in this article will be expressed, as
much as possible, in layman’s terminology to enable home inspectors to explain these issues to clients and agents in verbiage that they will understand. So let’s start with the basics.
As most inspectors know:
• A properly wired 120/240-volt subpanel includes a 4-conductor feeder; • Two of these lines are ungrounded conductors, commonly known as “hots;”
• One line is the grounded conductor, commonly knows as “the neutral;”
• And one line is the equipment grounding conductor, commonly known as “the ground.”
Understanding the rules and relationships that govern these last two lines, the neutral and the ground, is the
main subject of this article. Even among electricians, there are those who do not understand the fundamental
principles involved.
Rule #1: The grounded conductors (neutrals) and the equipment grounds must always be isolated after the
main service panel. There are two essential reasons for this rule:
A) To provide a low-resistance path for current travel to the transformer in the event of a ground fault.
This path should not be energized to ensure that it is fully available for the fault current in the event
of a malfunction.
B) To prevent the ground wires from conducting returning neutral current during normal operation.
Neutral current in the equipment ground wires can energize the casings and enclosures of appliances and equipment.
Rule #2: In a subpanel, the terminal bar for the grounded conductors (commonly known as the neutral bus)
should always be insulated from the enclosure. The reason for this rule is to prevent the enclosure from conducting current.
Rule #3: In a subpanel, the terminal bar for the equipment ground (commonly known as a ground bus)
should be bonded (electrically connected) to the enclosure. The reason for this rule is to provide a path to the
service panel and the transformer in case of a ground fault to the subpanel enclosure.
Rule #4: A terminal slot, no matter how large, can be assumed to be approved for one conductor only unless
it is labeled otherwise. However, Rule #5 is an exception.
Rule #5: The terminal slots in the equipment grounding bus are permitted to have one or two conductors of
the same size, per manufacturer’s instructions.
CASEY, O'MALLEY ASSOCIATES
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COA– Inspection
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Education
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Inspection Technical Tip
S U B PA N E L G RO U N D I N G B A S I C S
By Michael Casey & Barry Stone, COA—The Right Direction℠
The case of the married grounds and neutrals. This photo illustrates the most common of all
subpanel
grounding violations. The
grounded conductors and the
equipment
grounds are connected to the
same terminal
bar, contrary to
Rule #1. In the
event of a
ground fault, the
neutral wires can
conduct fault current. During normal operation,
the grounding
wires can energize equipment
enclosures and
casings with neutral current (yes,
the neutral carries current; the reason the conductors are insulated). A separate terminal bar,
bonded to the subpanel, is needed for the equipment grounding conductors. This repair should
be performed by a licensed electrical contractor.
CASEY, O'MALLEY ASSOCIATES
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Phone: 866-363-1330
E-mail: mike@CaseyOmalleyAssociates.com
Copyright © COA all rights reserved
COA– Inspection
Claims and
Education
Management
Inspection Technical Tip
S U B PA N E L G RO U N D I N G B A S I C S
By Michael Casey & Barry Stone COA—The Right Direction℠
The case of the bonded neutral bus. This photo illustrates another common subpanel violation -- bonding the
grounded
conductor
terminal bar
(neutral
bus) to the
subpanel
enclosure,
contrary to
Rule #2.
The copper
jumper from
the terminal
bar to the
body of the
panel enables the
neutral
wires to energize the panel enclosure or to conduct live current during a ground fault. The bond
jumper should be removed by a licensed electrical contractor.
CASEY, O'MALLEY ASSOCIATES
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Copyright © COA all rights reserved
COA– Inspection
Claims and
Education
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Inspection Technical Tip
S U B PA N E L G RO U N D I N G B A S I C S
By Michael Casey & Barry Stone COA—The Right Direction℠
The case of the stuffed terminal lug. In this photo we see a subpanel with the proper 4conductor
feeder. However, the installer made
an obvious
error at the
upper left corner. The
equipment
grounds are
bundled together and
stuffed into a
mechanical
connector approved for
one conductor only, violating Rule
#4. A grounded terminal bar is needed in this panel, with one or two wires to each terminal slot.
This panel is a candidate for further review and correction by a licensed electrical contractor.
CASEY, O'MALLEY ASSOCIATES
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Phone: 866-363-1330
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Copyright © COA all rights reserved
COA– Inspection
Claims and
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Inspection Technical Tip
S U B PA N E L G RO U N D I N G B A S I C S
By Michael Casey & Barry Stone COA—The Right Direction℠
The case of the split bolt connector. In this photo, the installer used a mechanical connector
to bond the equipment
grounding conductor to the
enclosure – with no provision for branch circuit
equipment grounds. The
branch circuit equipment
grounds were then bundled
with a bronze or copper
split bolt mechanical connector. There are three issues with this installation.
First, the branch circuit
equipment grounds are not
bonded with the feeder
equipment ground. Thus, there is no proper path back to the service equipment and the transformer for ground fault current. Second, split bolt connectors are generally used to tap a main
conductor and are approved for a maximum of two conductors only. This one contains a dozen
conductors or more. Third, bronze or copper split bolts are approved for copper conductors only.
Aluminum conductors are apparent in this bundle. Once again a licensed electrical contractor
should be recommended to make corrections.
This split bolt is approved
for copper wire only
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This split bolt is approved
for copper and aluminum
wire
PAGE 6
Phone: 866-363-1330
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Copyright © COA all rights reserved
COA– Inspection
Claims and
Education
Management
Inspection Technical Tip
S U B PA N E L G RO U N D I N G B A S I C S
By Michael Casey & Barry Stone COA—The Right Direction℠
The case of the mechanical terminal lug. In this photo, the requirement to bond the equipment grounds to the enclosure has been
met, and the path back to the service
equipment appears to exist, but the mechanical connector is not approved for
more than one conductor. An approved
terminal bar should be installed, with no
more than two wires to a terminal slot.
Again, this is a repair for a licensed electrical contractor.
In most of the cases cited in this article, the simple installation of a listed grounding terminal bar
would resolve the installation problems. Most of these are readily available for about $5 each.
CASEY, O'MALLEY ASSOCIATES
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Phone: 866-363-1330
E-mail: mike@CaseyOmalleyAssociates.com
Copyright © COA all rights reserved
COA– Inspection
Claims and
Education
Management
Inspection Technical Tip
S U B PA N E L G RO U N D I N G B A S I C S
By Michael Casey & Barry Stone COA—The Right Direction℠
One final grounding issue: We are often asked if subpanels added at spas and wells can be
fed with a three-conductor feeder (two hots and one neutral) and then “grounded” by adding a
grounding electrode (grounding rod). There are four reasons why this grounding method is not
acceptable:
1) The primary purpose of a grounding rod is to provide lightning protection. A rod is required for
separate buildings with subpanels, but not as a substitute ground fault path for the system.
2) A metal rod driven into the dirt does not provide a low-resistance path back to the service
equipment and transformer to clear ground faults. In order to clear a ground fault, the equipment
ground must be separate from the neutral and be continuous back to the service equipment, as
stated at the beginning of this article.
3) Dirt is not a good conductor of electricity, especially when it is dry.
4) Four-conductor feeders are required for all subpanels (even in detached buildings and structures) since the 2008 NEC.
Whenever grounding conditions appear questionable, don’t guess and don’t take chances. Recommend a licensed electrical contractor.
CASEY, O'MALLEY ASSOCIATES
www.CaseyOmalleyAssociates.com
www.inspectionconference.com
www.InspectionTrainingPros.com
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Phone: 866-363-1330
E-mail: mike@CaseyOmalleyAssociates.com
Copyright © COA all rights reserved
COA– Inspection
Claims and
Education
Management
Inspection Technical Tip
S U B PA N E L G RO U N D I N G B A S I C S
By Michael Casey & Barry Stone COA—The Right Direction℠
Michael Casey is a partner with Casey, O’Malley Associates; a national A.M. Best recommended consulting and
inspector training firm based in San Diego. Mike is a past president of the California Real Estate Inspection Association (1994/1995) and of the American Society of Home Inspectors (ASHI) (2002). He is multi-code certified by the
ICC and IAPMO. He is also a licensed general, plumbing and mechanical contractor in several states and a Virginia
Certified Home Inspector. Besides co-authoring several books in the Code Check series, Casey has authored many
other books, has taught home and building inspection and has an expert witness practice throughout North America
since 1987.
Michael can be contacted at: mike@caseyomalleyassociates.com
Barry Stone has practiced home inspections on the Central Coast of California since 1987 where he survives his
Realtor reputation as a “deal killer.” He is an ICC certified building inspector and mechanical inspector and an
AHERA certified asbestos inspector. Stone writes the nationally syndicated column “Ask the Inspector” and authored the book “The Consumer Advocates Guide to Home Inspection.”
Barry can be contacted at asktheinspector@charter.net
The information in this article is © 2011 Michael Casey and Barry Stone
CASEY, O'MALLEY ASSOCIATES
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COA– Inspection
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Education
Management